Hi-Res
VersionThis
artist's drawing view gives an impression of how common
planets are around the stars in the Milky Way. The planets,
their orbits and their host stars are all vastly magnified
compared to their real separations. A six-year search that
surveyed millions of stars using the microlensing technique
concluded that planets around stars are the rule rather than
the exception. The average number of planets per star is
greater than one.Credit:
ESO/M. Kornmesser

Over the past 16 years,
astronomers have detected more than 700 confirmed exoplanets and
have started to probe the spectra and atmospheres of these
worlds. While studying the properties of individual exoplanets is
undeniably valuable, a much more basic question remains: how
commonplace are planets in the Milky Way?

Most currently known exoplanets
were found either by detecting the effect of the gravitational
pull of the planet on its host star or by catching the planet as
it passes in front of its star and slightly dims it. Both of
these techniques are much more sensitive to planets that are
either massive or close to their stars, or both, and many planets
will be missed.

An international team of
astronomers has searched for exoplanets using a totally different
method — gravitational microlensing — that can detect
planets over a wide range of mass and those that lie much further
from their stars.

Arnaud Cassan (Institut
dʼAstrophysique de Paris), lead author of the Nature paper,
explains: "We have
searched for evidence for exoplanets in six years of microlensing
observations. Remarkably, these data show that planets are more
common than stars in our galaxy. We also found that lighter
planets, such as super-Earths or cool Neptunes, must be more
common than heavier ones."

The astronomers used
observations, supplied by the PLANET and OGLE teams, in which
exoplanets are detected by the way that the gravitational field
of their host stars, combined with that of possible planets, acts
like a lens, magnifying the light of a background star. If the
star that acts as a lens has a planet in orbit around it, the
planet can make a detectable contribution to the brightening
effect on the background star.

Jean-Philippe Beaulieu
(Institut d'Astrophysique de Paris), leader of the PLANET
collaboration adds: "The
PLANET collaboration was established to follow up promising
microlensing events with a round-the-world network of telescopes
located in the southern hemisphere, from Australia and South
Africa to Chile. ESO telescopes contributed greatly to these
surveys.”

Microlensing is a very powerful
tool, with the potential to detect exoplanets that could never be
found any other way. But a very rare chance alignment of a
background and lensing star is required for a microlensing event
to be seen at all. And, to spot a planet during an event, an
additional chance alignment of the planet’s orbit is also
needed.

Although for these reasons
finding a planet by microlensing is far from an easy task, in the
six year's worth of microlensing data used in the analysis,
three exoplanets were actually detected in the PLANET and OGLE
searches: a super-Earth, and planets with masses comparable to
Neptune and Jupiter. By microlensing standards, this is an
impressive haul. In detecting three planets, either the
astronomers were incredibly lucky and had hit the jackpot despite
huge odds against them, or planets are so abundant in the Milky
Way that it was almost inevitable.

The astronomers then combined
information about the three positive exoplanet detections with
seven additional detections from earlier work, as well as the
huge numbers of non-detections in the six year's worth of data —
non-detections are just as important for the statistical analysis
and are much more numerous. The conclusion was that one in six of
the stars studied hosts a planet of similar mass to Jupiter, half
have Neptune-mass planets and two thirds have super-Earths. The
survey was sensitive to planets between 75 million kilometers and
1.5 billion kilometers from their stars (in the Solar System this
range would include all the planets from Venus to Saturn) and
with masses ranging from five times the Earth up to ten times
Jupiter.

Combining the results suggests
strongly that the average number of planets around a star is
greater than one. They are the rule rather than the exception.

“We used to think
that the Earth might be unique in our galaxy. But now it seems
that there are literally billions of planets with masses similar
to Earth orbiting stars in the Milky Way,”
concludes Daniel Kubas, co-lead author of the paper.